The present invention relates to a selective excitation method and apparatus and a magnetic resonance imaging method and apparatus, and more particularly to a method and apparatus for selectively exciting spins of atomic nuclei, and a magnetic resonance imaging method and apparatus employing such a selective excitation apparatus.
In magnetic resonance imaging, gradient magnetic fields are generated in a space to be imaged to allow the three-dimensional position of voxels to be identified by frequency of spins of atomic nuclei, such as protons. The generation of the gradient magnetic fields involves generating a static magnetic field a having a uniform magnetic field strength B0 in a field of view (FOV), as exemplarily shown as a profile of the magnetic field strength in FIG. 1, applying a symmetric gradient magnetic field b having a direction on one side and a direction on the other side opposite to each other with respect to the center O of the static magnetic field, and obtaining a composite magnetic field c with a gradient by composing the magnetic fields a and b. To generate the static magnetic field a, a superconductive electromagnet, a normal conductive electromagnet, a permanent magnet or the like is employed. To generate the gradient magnetic field b, a gradient coil is employed having an appropriate loop shape.
Since the frequency of spins linearly varies with a distance Z in the FOV due to the composite magnetic field c having a gradient, a desired slice is selectively excited by appropriately choosing a frequency of an RF (radio frequency) excitation signal, to produce (i.e., reconstruct) a tomographic image based on magnetic resonance signals generated from the slice.
In generating a gradient magnetic field as above, the linearity of the gradient tends to be deteriorated as the distance becomes larger from the center O in the FOV. Although the linearity of the gradient may be improved by carefully designing the loop shape of the gradient coil, there has to be a trade-off between the linearity and other conditions, such as countermeasures against eddy current, and an ideal linearity cannot be always achieved. Accordingly, when a slice is selected at a position away from the center O (i.e., at an offset position), a different slice position from the desired one is excited by using an RF excitation frequency (offset frequency) determined by a nominal gradient magnetic field corresponding to the desired slice position.
It is therefore an object of the present invention to provide a selective excitation method and apparatus that is not affected by the non-linearity of a gradient magnetic field, and a magnetic resonance imaging method and apparatus employing such a selective excitation apparatus.
In accordance with a first aspect of the invention, there is provided a selective excitation method for selectively exciting spins of atomic nuclei within a subject to be imaged by an RF signal, the selective excitation being effected on a slice position defined at a distance predetermined in relative to a center of a magnetic field space having a gradient, the method comprising the step of employing as the RF signal an RF signal having a frequency corresponding to a slice position redefined differently from the defined slice position according to a non-linearity error in the gradient.
In accordance with a second aspect of the invention, there is provided a selective excitation method for selectively exciting spins of atomic nuclei within a subject to be imaged by an RF signal, the selective excitation being effected on a slice position defined at a distance predetermined in relative to a center of a magnetic field space having a gradient, the method comprising the step of adjusting the gradient so that the frequency of spins at the defined slice position equals the frequency of the RF signal.
In accordance with a third aspect of the invention, there is provided a selective excitation apparatus for selectively exciting spins of atomic nuclei within a subject to be imaged by an RF signal, the selective excitation being effected on a slice position defined at a distance predetermined in relative to a center of a magnetic field space having a gradient, the apparatus comprising RF excitation means employing as the RF signal an RF signal having a frequency corresponding to a slice position redefined differently from the defined slice position according to a non-linearity error in the gradient.
In accordance with a fourth aspect of the invention, there is provided a selective excitation apparatus for selectively exciting spins of atomic nuclei within a subject to be imaged by an RF signal, the selective excitation being effected on a slice position defined at a distance predetermined in relative to a center of a magnetic field space having a gradient, the apparatus comprising gradient adjusting means for adjusting the gradient so that the frequency of spins at the defined slice position equals the frequency of the RF signal.
(Effect)
According to the present invention, an RF signal having a frequency corresponding to a slice position redefined differently from a predefined slice position according to a non-linearity error in the gradient magnetic field is employed as an RF signal for exciting spins. Alternatively, the gradient of the magnetic field is adjusted so that the frequency of spins at the predefined slice position equals the frequency of the RF excitation signal.
Therefore, the present invention can provide a selective excitation method and apparatus that is not affected by the non-linearity of a gradient magnetic field, and a magnetic resonance imaging method and apparatus employing such a selective excitation apparatus.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.